As is known, Global Positioning System (GPS) is a worldwide radio-navigation system formed by a constellation of satellites and corresponding ground stations. Each satellite continually broadcasts its location in space along with the transmission time from an internal clock. GPS receivers are able to determine their position by receiving and analyzing signals transmitted from the GPS satellites.
In particular, for each signal from a GPS satellite, a GPS receiver computes the difference between the transmission time and the time at which the signal has been received and then, assuming GPS signal propagation speed to be known, computes its distance from the GPS satellite based on this difference. This distance is called pseudorange. A GPS receiver determines its location by performing a geometric triangulation on pseudoranges relating to several GPS satellites. Two-dimensional locations are able to be determined by exploiting pseudoranges relating to three GPS satellites, and three-dimensional locations are able to be determined by exploiting pseudoranges relating to four or more GPS satellites.
Although the current GPS has been successful, it has several shortcomings that affect the accuracy of positioning calculations. In fact, GPS satellite signals are subject to errors caused by ionospheric and tropospheric disturbances, satellite clock drifts and satellite orbit discrepancies. For example, ionospheric and tropospheric refraction can slow satellite signals and cause carrier signals and codes to diverge. Because ionospheric disturbances vary greatly from location to location, these errors are difficult to correct with civilian-type GPS receivers.
These shortcomings make GPS in itself not useable for safety critical services, such as air navigation. In fact, for instance, GPS signal is affected by too much error and uncertainty to meet mandatory air aviation precision requirements.
For this reason, recently, different GPS augmentation techniques have been developed, i.e. techniques oriented to improve accuracy, reliability, availability, and integrity of GPS through the integration of external information into the calculation process.
In particular, a Satellite Based Augmentation System (SBAS) is a system that supports wide-area or regional augmentation through the use of additional satellite-broadcast messages. Examples of SBASs are the Wide Area Augmentation System (WAAS) developed in the U.S.A. by the Federal Aviation Administration (FAA), the European Geostationary Navigation Overlay Service (EGNOS) developed in Europe by the European Space Agency (ESA), the European Commission and EUROCONTROL, and the Japanese Multi-functional Satellite Augmentation System (MSAS). FIG. 1 shows schematically locations of several SBASs under development in the world.
Radio Technical Commission for Aeronautics (RTCA) document number DO-229D, called Minimum Operational Performance Standards (MOPS) for Global Positioning System/Wide Area Augmentation System Airborne Equipment, and prepared by RTCA Special Committee 159, contains Minimum Operational Performance Standards (MOPS) for airborne navigation equipment (2D and 3D) using the GPS augmented by the WAAS and represents international SBAS standard to be applied by all government agencies. In this way all SBAS service providers will ensure signal compatibility and system interoperability thus contributing to a true worldwide seamless navigation service.
Specifically, a receiver compliant with RTCA/DO-229D MOPS will work with any SBAS, i.e. WAAS, EGNOS, MSAS etc.
In detail, SBASs improve the performances of GPS with the objective to make it useable for safety critical services, such as air navigation. This is accomplished by providing, by means of separate signals, a set of corrections that improve the accuracy of the position calculation performed by the user satellite receiver. In particular, EGNOS provides these corrections not only for GPS but also for the Global Orbiting Navigation Satellite System (GLONASS).
Generally speaking, an SBAS is based on the principle of the spatial and temporal correlation of measurement errors that arise when making distance measurement from a space born source. The difference between the theoretical and the real measurement performed in a known position can be found, with similar values, in other real measurements performed in the nearby of the known position. In other words this principle says that the distance measurements made in a small geographical area may be affected by the same errors. So, once you know the measurement error in one place, it can be used as a correction for the distance measurements made in nearby places. In a scenario where several reference points are available, a wide area correlation law, which models the difference in distance measurements, can be derived. These data collected by a network of reference stations are processed and then transmitted to the users, by means of geostationary satellites, on a signal having the same frequency as GPS (L1=1575.42 MHz) and a different data format. SBAS messages contain information for the computation of pseudoranges corrections, but also integrity parameters, used to estimate the degree of confidence of the position computation.
The information contained in the navigation message modulated on L1, the additional ranging capability offered by the geostationary satellites and the complexity of ground processing and checks, are able to improve the accuracy, the integrity and the reliability of GPS.
As said above and as all SBASs, EGNOS has been designed to meet the demanding air navigation performance requirements, in particular thought for landing aircrafts:                accuracy will be improved to about 2-4 meters vertically and 1-3 meters horizontally through the broadcast of Wide-Area Differential (WAD) corrections;        integrity (safety) will be improved both through the high degree of redundancy in the system and by alerting users within 6 seconds if some system degradation occurs to EGNOS, GPS or GLONASS;        continuity will be improved in order to keep the system working during the next 150 seconds from the beginning of any intended operation; and        availability will be improved by broadcasting GPS look-alike signals from geostationary satellites.        
EGNOS measurements already confirm that accuracy will be in the order of 2-4 meters vertically and 1-3 meters horizontally inside the European Civil Aviation Conference (ECAC) area.
More in detail, EGNOS provides a European-wide, standardized and quality-assured positioning system suitable for a diverse range of applications. It is highly compatible with GPS, so a single antenna and receiver may process both the GPS and EGNOS signals eliminating the need for a separate radio to receive corrections.
FIG. 2 shows schematically EGNOS architecture. In particular, thirty-four Reference and Integrity Monitoring Stations (RIMS) are deployed to monitor the satellite constellation satellites. Each satellite has to be monitored by multiple RIMS before correction and integrity messages are generated. Four Mission Control Centres (MCC) process data from these RIMS to generate the corrections and integrity messages for each satellite. In particular, MMCs generate a single set of integrity data and GPS corrections for Europe comprising terms to correct for each satellite clock and ephemeris errors as well as errors due to the ionosphere.
Only one of these MCCs is active and operational, the other MCCs are hot spares that can be activated if a problem occurs.
Navigation Land Earth Stations (NLES) upload the corrections and integrity messages to the satellites, for onward broadcast to the users. The system will deploy two NLESs (one primary and one backup) for each of the three geostationary satellites, and a further NLES for test and validation purposes.
The EGNOS space segment is composed of three geostationary satellites with global earth coverage. The EGNOS operational system is based on the use of two INMARSAT-3 satellites (AOR-E and IOR), as well as the ESA ARTEMIS satellite.
The integrity data and corrections are modulated on a GPS look-alike signal and broadcasted to users from the three geostationary satellites. The resulting performance (close to 1 m across Europe) is independent of user/reference station distance. EGNOS users will benefit from enhanced availability due to the three additional ranges.
Moreover, EGNOS users should be able to track at least two geostationary satellites. It takes less than six seconds to notify users about a problem with one of the satellite constellations once it has been monitored by the RIMS network.
EGNOS provides different levels of service at different parts of the area covered by the geostationary satellites. Optimum performance is obtained within the core coverage area, as shown in FIG. 2. There is degraded performance outside the core area, although there is some potential for improvement through interoperability with the Japanese, American and Canadian systems.
As previously said, EGNOS uses the same frequency (L1) and ranging codes as GPS, but has a different data message format. The messages come once per second and are made up of 250 bits, among which 212 represent augmentation data, eight are used for acquisition and synchronization, six to identify the message type and the remaining twenty four are parity bits to protect against the use of corrupted data.
Sixty-four different message types have so far been defined to broadcast integrity data and corrections and some of them are showed in the following table:
TypeCommentTypeComment0Don't use this SBAS signal17GEO satellite almanacsfor safety applications1PRN Mask assignments18Ionospheric grid point masks2-5Fast corrections24Mixed fast/slow errorcorrections6Integrity information25Slow satellite error corrections7Fast correction26Ionospheric delay correctionsdegradation factor9GEO navigation message27SBAS service message10 Degradation parameters63Null message12 SBAS networkTime/UTC offsets
The message schedule follows a 6-second duty cycle. This is structured both to prioritize the 6-second integrity time-to-alarm and to minimize the time for EGNOS initialization. However, although EGNOS message schedule has been thought to minimize initialization period, EGNOS receivers still need to be initialized during a non-operative initialization period before being active, i.e. before providing corrections.
FIG. 3 shows schematically EGNOS message providing and scheduling architecture.
Fast and slow corrections model the temporal decorrelation of the different error sources. The fast corrections model rapidly changing error sources including satellite clock errors. The slow corrections model more slowly changing error sources including long-term satellite clock drift, satellite ephemeris errors and ionospheric delays, the latter provided at pre-defined grid points.
Message processing is complex both because the messages have been designed to minimize the bandwidth requirements and because they need to account for updated GPS navigation information.
Hereinafter two examples of SBAS message processing are given: the former relating to a fast correction message and the latter relating to a slow correction message.
In particular, SBAS message type 2, as shown in the previous table, contains fast corrections. At the arrival of a SBAS message of this type, the EGNOS receiver performs the following operations:                the SBAS message is decoded and pseudorange correction PRC, range rate correction RRC, and applicability time T0 are extracted;        difference TR-T0 is computed, where TR is the instant at which the EGNOS message 2 has been received and for fast corrections always results that T0<TR;        actual pseudorange correction PRCA is computed as follows:PRCA=PRC+RRC×(TR−T0)        the actual pseudorange correction PRCA is added to the GPS-based computed pseudorange to remove fast satellite clock error and to obtain a corrected pseudorange; and        augmented position is computed based on the corrected pseudorange.        
Contrary to SBAS message type 2, SBAS message type 25 contains slow corrections. At the arrival of a SBAS message of this type, the EGNOS receiver performs the following operations:                the SBAS message is decoded and considered valid while data extracted from the previous message 25 are still kept in memory; and        velocity code v, satellite location correction terms Δx, Δy, Δz, and time correction terms are extracted.        
At this point, the EGNOS receiver may perform two alternative sets of operations depending on value assumed by the velocity code v.
If v=0,                the previous message 25 is substituted by the actual one;        GPS-received satellite location (xsatGPS,ysatGPS,zsatGPS) is corrected as follows:        
         {                                                      x              sat              corr                        =                                          x                sat                GPS                            +                              Δ                ⁢                                                                  ⁢                x                                                                                                    y              sat              corr                        =                                          y                sat                GPS                            +                              Δ                ⁢                                                                  ⁢                y                                                                                                    z              sat              corr                        =                                          z                sat                GPS                            +                              Δ                ⁢                                                                  ⁢                z                                                        and                augmented position is computed based on the corrected satellite location (xsatcorr,ysatcorr,zsatcorr).        
If v=1,                satellite velocity correction terms Δvx, Δvy, Δvz and applicability time T0 are also extracted;        if TR<T0, where TR is the instant at which the EGNOS message 25 has been received, and the previous message 25 is still valid, corrections of the previous message 25 are applied;        if TR<T0 and the previous message 25 is no longer valid, difference TR-T0 is computed based on the actual message 25;        if TR>T0 difference TR-T0 is computed;        final satellite location correction terms are computed as follows:        
         {                                                      Δ              ⁢                                                          ⁢                              x                F                                      =                                          Δ                ⁢                                                                  ⁢                x                            +                              Δ                ⁢                                                                  ⁢                                  v                  x                                ×                                  (                                                            T                      R                                        -                                          T                      0                                                        )                                                                                                                    Δ              ⁢                                                          ⁢                              y                F                                      =                                          Δ                ⁢                                                                  ⁢                y                            +                              Δ                ⁢                                                                  ⁢                                  v                  y                                ×                                  (                                                            T                      R                                        -                                          T                      0                                                        )                                                                                                                    Δ              ⁢                                                          ⁢                              z                F                                      =                                          Δ                ⁢                                                                  ⁢                z                            +                              Δ                ⁢                                                                  ⁢                                  v                  z                                ×                                  (                                                            T                      R                                        -                                          T                      0                                                        )                                                                                        GPS-received satellite location (xsatGPS,ysatGPS,zsatGPS) is corrected as follows:        
         {                                                      x              sat              corr                        =                                          x                sat                GPS                            +                              Δ                ⁢                                                                  ⁢                                  x                  F                                                                                                                    y              sat              corr                        =                                          y                sat                GPS                            +                              Δ                ⁢                                                                  ⁢                                  y                  F                                                                                                                    z              sat              corr                        =                                          z                sat                GPS                            +                              Δ                ⁢                                                                  ⁢                                  z                  F                                                                        and                augmented position is computed based on the corrected satellite location (xsatcorr,ysatcorr,zsatcorr).        